Ocular Ultrasonography and Effect of Topically Applied Rocuronium Bromide in Louisiana Pine Snakes (Pituophis ruthveni)
Louisiana pine snakes (Pituophis ruthveni) are a threatened species with a range limited to portions of Louisiana and Texas. There are captive reproduction programs that aim to assist in the recovery of this species. Therefore, obtaining information that can aid in maintaining their health, such as a complete ocular examination, is a critical component in conservation efforts. Examination of the snake ocular posterior segment is challenging because of the presence of striated uveal musculature that is unresponsive to anticholinergic agents, and the presence of the spectacle, which may create a barrier to topical mydriatics. The objectives of this study were to document normal ocular biometric measurements (axial globe length [AP], lens thickness (LT), and vitreous chamber depth [VCD]) in this species using ultrasonography (US) and to assess the effects of topically applied rocuronium bromide (RB) solution (one- and two-drop protocol, 20 μl and 40 μl, respectively) to facilitate noninvasive examination of the fundus. The ocular biometric measurements were then correlated to morphometric data including snout–vent length (SVL), tail length (TL), total body length (SVL + TL), sex, and body condition score (BCS). We found that US, using an 8–18-MHz linear array probe, provided adequate image quality to measure the above parameters in all snakes (n = 8), and that there were no significant correlations between the morphometric data and ocular biometric US measurements (P > 0.05). RB solution applied topically to the healthy, intact spectacle did not induce mydriasis in a subset of this same population (n = 6; P > 0.05) using either the one- or two-drop protocol. In conclusion, ocular US can be utilized as a noninvasive diagnostic tool in Louisiana pine snakes and further studies are needed to evaluate noninvasive methods of inducing mydriasis in this species.Abstract
(A) A representative B-mode vertically oriented ultrasound image of the ocular contents of a Louisiana pine snake (Pituophis ruthveni). The anterior aspect of the spectacle, anterior lens capsule, posterior lens capsule, and posterior aspect of the globe are readily visualized using an 8–18 MHz linear array probe transducer on an Ultrasound GE Logiq unit and are labeled for clarity. (B) The same image in triplicate with red lines illustrating the three measurements taken on each eye. The left image shows the axial globe length (AP) or the distance from the anterior aspect of the spectacle to the posterior pole of the globe. The middle image shows the lens thickness (LT) or the distance between the anterior lens capsule to posterior lens capsule. The right image shows the depth of the vitreous chamber (VCD) or the distance between the posterior lens capsule and the posterior pole of the globe.
An image of a Louisiana pine snake (Pituophis ruthveni) being photographed to assess for mydriasis following topical application of rocuronium bromide (RB) to the spectacle. To improve contrast and reduce artifacts that could impact pupil measurements, a Nikon camera fitted with an infrared filter (720 nm) was used to capture all images for analysis. Snakes were treated with either topically applied RB solution or saline on the spectacle and then photographed at 10-, 20-, 30-, 40-, and 60-min intervals. Schirmer tear test strips were taped to the face of subjects, and at perpendicular angles to each other, and used to provide distance to calibrate ImageJ (Available at: rsb.info.nih.gov/nih-image) and thus, measure pupil diameter. A 5-mm measurement on the Schirmer tear test strip was calibrated to pixels on each image, which was used to measure the rostral to caudal pupil diameter. The software then automatically converted the number of pixels to a millimeter distance.
Recorded pupil diameter in millimeter for control eyes of Louisiana pine snakes (Pituophis ruthveni) that received one drop of saline on the spectacle (blue circles) and treated eyes that received one drop of rocuronium bromide (RB) on the spectacle (red squares) over each time point. Time 0 represents the baseline pupil measurement before any saline or RB was administered.
Recorded pupil diameter in millimeters for control eyes of Louisiana pine snakes (Pituophis ruthveni) that received two drops of saline on the spectacle (blue circles) and treated eyes that received two drops of rocuronium bromide (RB) on the spectacle (red squares) over each time point. Time 0 represents the baseline pupil measurement before any saline or RB was administered.
An image of a Louisiana pine snake (Pituophis ruthveni) held in right lateral recumbency with one drop of rocuronium bromide (RB) applied topically to the left spectacle. Note the formation of a spherical droplet on the spectacular skin indicating the hydrophobic nature of the keratinized layer of the spectacle. This image was taken with a Nikon camera fitted with an infrared filter (720 nm).
Contributor Notes
Present address: Nashville Zoo at Grassmere, Nashville, TN 37211, USA
